The active-ray approach to rendering on distributed memory multiprocessors

Object dataflow is a popular approach used in parallel rendering. The data representing the 3D scene is statically distributed among processors and objects are fetched and cached only on demand. Most previous object dataflow methods were implemented on shared memory architectures and exploited spatial coherency to reduce hardware cache misses. We propose an efficient model for object dataflow parallel volume rendering on message passing machines. The active ray tracing algorithm is introduced and its ray storage mechanism is used to support latency hiding by postponing computation on inactive rays. Memory usage is optimized by letting objects migrate and replicate at different processors rather than the common static assignments. Our cache-only-memory approach uses a distributed-directory scheme to trace the location of objects at other nodes. A mechanism to minimize network congestion was implemented which optimizes channel utilization. Unlike previous methods, our approach can benefit from temporal coherence and effectively minimizes communication costs in successive frames. We implemented a volume ray casting instance of the algorithm on the Cray T3D and achieved higher efficiency and scalability than existing algorithms. We achieve interactive frame rates of approximately 20 Hz for 128³ volume, and 4 Hz for 256³ volume on 128 processors